1. Field of the Invention
This invention relates to a heat fixing assembly, and a fixing belt, of image forming apparatus such as electrophotographic apparatus and electrostatic recording apparatus, and to an endless metallic belt used in these.
2. Related Background Art
In image forming apparatus, heat roller type or belt heating type assemblies have been in wide use as fixing assemblies with which unfixed images (toner images) of intended image information which have been formed and held on recording mediums (such as transfer material sheets, electrofax sheets, electrostatic-recording paper, OHP sheets, printing paper and format paper) by a transfer method or a direct method at zone where an image formation process such as an electrophotographic process, electrostatic recording process or magnetic recording process is carried out are heat-fixed to the recording medium surfaces as permanently fixed images.
As materials for belts in such a belt heating method, heat-resistant resins are used. In particular, polyimide resins are used, which have superior heat resistance and strength. However, such resin belts have insufficient strength for meeting a demand for further high speed and high durability. Accordingly, it is proposed to use a belt made of a metal such as SUS stainless steel, titanium, nickel or the like as a base material.
Seamless (endless) belt base materials made of SUS stainless steel are obtained by plastic working such as spinning (see Japanese Patent Application Laid-open No. 2001-225134). In general, belts made of SUS stainless steel which are obtained by plastic working (such as rolling, drawing or spinning) stand unable to deal with the achievement of smaller diameter of endless fixing belts (18 mm or less in diameter) and the achievement of smaller wall thickness of fixing belt base materials (15 μm or less in thickness), as required in small-sized, high-speed and highly durable fixing assemblies. Then, because of a difference in stress distribution between the lengthwise direction and the circumferential direction, there is a possibility of causing cracks gathered in one direction. There are also limits for the length, diameter, thickness and dimensional precision of belts obtainable by the plastic working.
Seamless (endless) belt base materials made of nickel are commonly produced by electroforming making use of a nickel sulfamate bath or a nickel sulfate bath (see Japanese Patent Application Laid-open No. H09-034286). As electroformed nickel materials used in fixing assemblies of a high-frequency electromagnetic induction heating system, electroformed nickel materials are used which are usual gloss electroformed nickel materials improved in heat deterioration resistance (see Japanese Patent Application Laid-open No. 2002-258648).
On the sliding surface of such an electroformed nickel belt, a sliding layer formed of a polyimide or the like is provided in some cases in order to supplement wear resistance. However, what are called resin type materials including polyimides have thermal resistance which is larger by about 300 times that of the base material nickel, and hence require a long fixing rise time to loose an advantage the well heat-conducting nickel materials have. For single-metallic electroformed belts, it is difficult to have properties which satisfy all requirements of wear resistance, heat resistance, flexing resistance and durability. Combination of various metallic elements makes it possible to obtain electroformed belts having better properties.
A technique is proposed in which at least one of metallic elements belonging to Group 2, Group 3, Group 4 and Group 5 of the periodic table is incorporated in a proportion of from 10 to 10,000 ppm as mass fraction to control the growth of nickel plating crystals to make the crystals grow orderly so that the crystals of nickel may strongly be oriented to the (200) crystal plane, and also crystal transition temperature is raised to improve heat aging resistance and durability of the electroformed belts. However, there has still been room for improvement with regard to wear resistance, flexing resistance, heat resistance and durability which are required in fixing belts for compact and high-speed fixing assemblies (see Japanese Patent Application Laid-open No. 2002-241984).
In a nickel alloy containing 5% by mass or more of a different metal(s) other than nickel, half widths of diffraction peaks of the (111) crystal plane and (200) crystal plane in an X-ray diffraction pattern of a metallic layer are both made to be 0.5 to 2.0 by defining the half widths, and this enables securement of sufficient wear resistance and durability (see Japanese Patent Application Laid-open No. 2005-165291). However, the nickel alloy may inevitably have a low thermal conductivity because the different metal(s) is/are in an amount of as large as 5% by mass or more, or may inevitably have a low electromagnetic induction heating efficiency when applied in fixing assemblies of the high-frequency electromagnetic induction heating system. Accordingly, it has been necessary for the different metal(s) to be contained in a small quantity.